High-strength heat resisting cast steel, method of producing the steel, and applications of the steel

ABSTRACT

A high-strength heat resisting cast steel which has high creep rupture strength at temperatures of 620° C. or above, high toughness, and good weldability. A method of producing the steel, a steam turbine casing, a main steam valve casing, and a steam control valve casing, each casing being made of that steel, as well as a steam turbine power plant using those components are also provided. The high-strength heat resisting cast steel contains 0.06-0.16% by mass of C, 0.1-1% of Si, 0.1-1% of Mn, 8-12% of Cr, 0.1-1.0% of Ni, 0.7% or less of Mo, 1.9-3.0% of W, 0.05-0.3% of V, 0.01-0.15% of one or more of Nb, Ta and Zr in total, 0.1-2% of Co, 0.01-0.08% of N, and 0.0005-0.01% of B, the balance being Fe and unavoidable impurities.

This application is a continuation of U.S. patent application Ser. No.11/528,439, filed Sep. 28, 2006, which claims priority to JapanesePatent Application No. 2005-283199, filed Sep. 29, 2005 and which isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-strength heat resisting caststeel which has high creep rupture strength at temperatures of 620° C.or above and good weldability, and which is suitable for use in high-and intermediate-pressure inner casings of an ultra super critical (USC)pressure steam turbine and respective casings of a main steam valve anda steam control valve used in the USC pressure steam turbine in whichtemperature and pressure of main steam are not lower than 620° C. and 25MPa, respectively. Also, the present invention relates to a method ofproducing that steel, a steam turbine casing, a main steam valve and itscasing, and a steam control valve and its casing, each casing being madeof that steel, as well as a steam turbine power plant using thosecomponents.

2. Description of the Related Art

In known steam turbines with steam temperatures of 600° C. or below,CrMoV low-alloy cast steel and 11CrMoVNb cast steel are used as casingmaterials. Meanwhile, taking into account not only exhaustion of fossilfuel, such as petroleum and coal, but also the necessity of energysaving, an improvement in power generation efficiency of the steamturbine is demanded. Because increasing the steam temperature andpressure is the most effective measure for realizing the improvement inpower generation efficiency, it is tried to increase the steamtemperature in a thermal power plant. However, the currently employedcasing materials are not sufficient in strength when used as materialsof a more efficient turbine adapted for that purpose, and materialshaving higher strength are required.

The high-temperature strength of the above-mentioned materials is notsufficient for use in a casing of a high-temperature steam turbine withsteam temperature of 620° C. or above. Patent Document 1 (JP,A 7-118812)discloses a casing made of 9%-Cr steel, but the disclosed casing has avariation in high-temperature strength. In the known typical steamturbine, the steam temperature is 600° C. and the steam pressure is 25MPa. As rotor materials, there are known 1Cr-1Mo-1/4V ferrite-baselow-alloy forged steel and 11Cr-1Mo—V—Nb—N forged steel. As casingmaterials, there are known 1Cr-1Mo-1/4V ferrite-base low-alloy caststeel and 11Cr-1Mo—V—Nb—N cast steel. In particular, as examples havinghigher high-temperature strength among those materials, an austenitealloy is disclosed in Patent Document 2 (JP,A 62-180044), and martensitesteels are disclosed in Patent Document 3 (JP,A 2-290950), PatentDocument 4 (JP,A 4-371551), and Patent Document 5 (JP,A 9-59747).

SUMMARY OF THE INVENTION

Patent Document 5 discloses the martensite steel as a material havinghigher high-temperature strength than the above-mentioned known casingmaterials. However, the disclosed martensite steel is not sufficient instrength when used for the casing of the steam turbine with steamtemperature of 620° C. This leads to the problem that the thickness ofthe casing must be increased and larger thermal stress is caused whenthe turbine is started and stopped. Further, although Patent Document 5discloses materials of the rotor and the casing, etc., it pays noconsideration to a steam turbine and a thermal power plant systemadapted for an increased level of temperature.

An object of the present invention is to provide a high-strength heatresisting cast steel which has high creep rupture strength attemperatures of 620° C. or above, high toughness and good weldability, amethod of producing the steel, a steam turbine casing, a main steamvalve and its casing, and a steam control valve and its casing, eachcasing being made of that steel, as well as a steam turbine power plantusing those components.

To achieve the above object, the present invention provides ahigh-strength heat resisting cast steel containing 0.06-0.16% by mass ofC, 0.1-1% of Si, 0.1-1% of Mn, 8-12% of Cr, 0.1-1.0% of Ni, 0.7% or lessof Mo, 1.9-3.0% of W, 0.05-0.3% of V, 0.01-0.15% of one or more of Nb,Ta and Zr in total, 0.1-2% of Co, 0.01-0.08% of N, and 0.0005-0.01% ofB, the balance being Fe and unavoidable impurities. Also, the presentinvention provides a steam turbine casing, a main steam valve and itscasing, and a steam control valve and its casing, each casing being madeof that steel.

In the present invention, preferably, the high-strength heat resistingcast steel contains 0.0005-0.04% by mass of At and 0.02% or less of 0.Also, in orthogonal coordinates expressed by the relationship between[W/(Mo+0.5W)] and (Co/W), values of [W/(Mo+0.5W)] and (Co/W) are notlarger than the values represented by linear lines interconnecting acoordinate point A (1.1, 0.90), a coordinate point B (1.5, 0.55), and acoordinate point C (1.8, 0.55).

Further, preferably, the high-strength heat resisting cast steelcontains at least one of 1.5% by mass or less of Re, 0.5% or less of Nd,and 1.0% or less of Sr.

Moreover, preferably, the high-strength heat resisting cast steel hasgood weldability with such properties that creep rupture strength at620° C. and 10⁵ hours is 98 MPa or more, and impact absorbed energy atroom temperature is 29.4 J or more. To ensure higher reliability,preferably, the creep rupture strength at 620° C. and 10⁵ hours is 108MPa or more, and the impact absorbed energy at room temperature is 31.4J or more.

The method of producing the high-strength heat resisting cast steel,according to the present invention, comprises the steps of smelting, inan electric furnace, raw materials with the above-described composition;deaerating the smelted materials by vacuum ladle refining; and castingthe deaerated materials into a sand mold. Further, the method ofproducing the high-strength heat resisting cast steel comprises thesteps of annealing, at 1000-1150° C., the cast steel obtained after theabove-described casting step; performing thermal normalizing of theannealed steel through steps of heating to 1000-1100° C. and subsequentquick cooling; and performing two successive stages of heat treatment,i.e., primary tempering at 550-750° C. and secondary tempering at670-770° C.

The steam turbine casing, the main steam valve and its casing, and thesteam control valve and its casing, according to the present invention,are produced by using the high-strength heat resisting cast steel whichhas the above-described composition and is produced by theabove-described method. The steam turbine power plant according to thepresent invention is constituted by the steam turbine casing, the mainsteam valve, and the steam control valve. The reasons why respectiveingredient elements of the high-strength heat resisting cast steelaccording to the present invention are limited to the above ranges willbe described below.

C is an element that is required to be 0.06% or more to obtain hightensile length. However, if the C content exceeds 0.16%, the metalstructure becomes unstable when exposed to high temperatures for a longtime, thus resulting in reduction of long-time creep rupture strength.For that reason, the C content is limited to 0.06-0.16%. In particular,a preferable range is 0.08-0.14% and a more preferable range is0.09-0.12%.

Mn is added as a deoxidizer. By adding 0.1% of Mn, the effect of thedeoxidizer is achieved. Addition of Mn in excess of 1% reduces the creeprupture strength. In particular, a preferable range is 0.3-0.8% and amore preferable range is 0.4-0.7%.

Si is also added as a deoxidizer. Addition of Si can be reduced byemploying the steel-making technology based on, e.g., the vacuum carbondeoxidation method, but Si is required to be added 0.1% or more. Also,because Si generates the harmful δ ferrite structure, the addition of Siin excess of 1% must be avoided. Reducing the Si content has the effectof preventing generation of the harmful δ ferrite structure.Accordingly, when Si is added, the Si content is required to be held 1%or less. In particular, a preferable range is 0.6% or less and a morepreferable range is 0.2-0.6%.

Cr is effective in improving high-temperature strength andhigh-temperature oxidation. However, addition of Cr in excess of 12%causes the generation of the harmful δ ferrite structure, and excessiveaddition of Cr causes reduction of toughness. On the other hand, if theCr content is less than 8%, oxidation resistance againsthigh-temperature and high-pressure steam is not sufficient. Inparticular, a preferable range is 9.5-11.5% and a more preferable rangeis 10.0-11.0%.

Ni is an element that suppresses generation of δ ferrite and givestoughness. Therefore, the Ni content is required to be 0.1% at minimum.However, addition of Ni in excess of 1% reduces the creep rupturestrength at temperatures of 620° C. or above. For that reason, the Nicontent is limited to 0.1-1.0%. In particular, a preferable range is0.2-0.8% and a more preferable range is 0.3-0.7%.

W is effective in noticeably increasing the high-temperature andlong-time strength. If the W content is less than 1.9%, the effect ofincreasing the strength is not sufficient as heat resisting cast steelwhen the steel is used at temperatures of 620° C. or above. On the otherhand, if the W content exceeds 3.0%, toughness is reduced. Inparticular, a preferable range is 1.95-2.7% and a more preferable rangeis 2.0-2.5%.

Mo is added to increase the high-temperature strength. However, when Wis contained in excess of 1.9% as in the cast steel of the presentinvention, addition of Mo in excess of 1% reduces toughness and fatiguestrength. Accordingly, the Mo content is limited to 1.0% or less. Inparticular, a preferable range is 0.15-0.7% and a more preferable rangeis 0.2-0.6%.

V is effective in increasing the creep rupture strength. If the Vcontent is less than 0.05%, the reslting effect is not sufficient.Conversely, if the V content exceeds 0.3%, δ ferrite is generated toreduce the fatigue strength. In particular, a preferable range is0.10-0.25% and a more preferable range is 0.12-0.23%.

Nb is a very effective element to increase the high-temperaturestrength. However, excessive addition of Nb generates coarse eutectic NBcarbides, particularly in a large-sized steel ingot, thus causingprecipitation of δ ferrite that reduces the high-temperature strengthand the fatigue strength. For that reason, the Nb content is required tobe held less than 0.15%. Conversely, if the Nb content is less than0.01%, the resulting effect is not sufficient. In the case of alarge-sized steel ingot, in particular, a preferable range is 0.02-0.12%and a more preferable range is 0.04-0.10%.

Ta and Zr are effective in increasing low-temperature toughness. Asufficient effect is obtained by adding 0.15% or less of Ta and 0.1% orless of Zr solely or in combination. When Ta is added 0.1% or more,addition of Nb can be omitted.

Co noticeably increases the high-temperature strength when added 0.1% ormore. Such effect is presumably attributable to the interaction with W.In other words, that effect is a phenomenon specific to the alloy of thepresent invention which contains 1.9% or more of W. On the other hand,excessive addition of Co over 2% reduces structure stability at hightemperatures and hence reduces the creep rupture strength. Therefore,the Co content is limited to 0.1-2%. In particular, a preferable rangeis 0.1-1.6% and a more preferable range is 0.2-1.2%.

N has the actions of not only precipitating a nitride of V, but alsoincreasing the high-temperature strength in the solid solution statebased on the IS effect (interaction between an interstitial solidsolution element and a substitutive solid solution element) incooperation with Mo and W. Therefore, N is required to be added 0.01% atminimum. However, addition of N in excess of 0.08% reduces ductility.For that reason, the N content is limited to 0.01-0.08%. In particular,a preferable range is 0.015-0.075% and a more preferable range is0.015-0.06%.

Al is added 0.0005% or more as a deoxidizer and a crystal grain reducingagent (refiner). However, At is a strong nitride-forming element andfixates N that effectively acts to prevent creep. Particularly, additionof Al in excess of 0.04% reduces the long-time creep strength at 10⁴hours or more in a high temperature range. Also, Al promotesprecipitation of the Laves phase in the form of a brittle intermetalliccompound made of mainly W, thus causing precipitation of the Laves phaseat the crystal grain boundary and reduction of the creep rupturestrength at the longer-time side. In the case of excessive crystal grainrefinement, in particular, the Laves phase is continuously precipitatedalong the crystal grain boundary. Accordingly, an upper limit of the Alcontent is set to 0.04%. In particular, a preferable range is0.001-0.035% and a more preferable range is 0.003-0.030%.

B is effective in increasing the high-temperature strength by the actionof strengthening the crystal grain boundary and the action of causingsolid solution in M₂₃C₆ to thereby prevent aggregation of M₂₃C₆-typecarbides into coarser grains. That effect is obtained by adding 0.0005%of B at minimum. However, addition of B in excess of 0.01% impedesweldability. For that reason, the B content is limited to 0.0005-0.01%.In particular, a preferable range is 0.001-0.008% and a more preferablerange is 0.002-0.007%.

The O content in excess of 0.015% reduces the high-temperature strengthand toughness values, it should be kept 0.020% or less. In particular, apreferable range is 0.015% or less and a more preferable range is 0.010%or less.

In addition, it was experimentally found that Mo, W and Co among theabove-mentioned ingredients greatly affect the high-temperature strengthand the high-temperature structure stability and cause the action in acombined way in the steel of the present invention.

More specifically, to obtain material characteristics having both thehigh-temperature strength and the high-temperature structure stabilityat 620° C. or above, it is preferable, in an addition ratio of Mo and Wwhich are effective in increasing the creep strength, to increase the Wratio for noticeably increasing the long-time and high-temperaturestrength, and to reduce an addition ratio of Co and W (i.e., Co/W) forincreasing the high-temperature structure stability. Stated another way,in the orthogonal coordinates expressed by [W/(Mo+0.5W)] and (Co/W) andrepresenting the relationship among Mo, W and Co, values of[W/(Mo+0.5W)] and (Co/W) are preferably included in a region locatedunder linear lines ABC interconnecting the coordinate point A (1.1,0.90), the coordinate point B (1.5, 0.55), and the coordinate point C(1.8, 0.55).

Re noticeably increases the high-temperature strength by strengtheningwith the solid solution. Because excessive addition promotesembrittlement, Re is preferably added 2% or less. In consideration of Rebeing a rare element, a preferable range is 1.5% or less and a morepreferable range is 1.2% or less from the practical point of view.

Nd increases the high-temperature strength by forming carbo-nitrides. Ndexhibits an effect even when added in small amount, and excessiveaddition promotes embrittlement. Therefore, Nd is preferably added 1% orless. In consideration of Nd being a rare element, a preferable range is0.5% or less and a more preferable range is 0.3% or less from thepractical point of view.

Sr strengthens the old austenite grain boundary and increases thelow-temperature toughness and strength, thus increasing especially thecreep rupture strength. However, excessive addition promotes formationof carbo-nitrides at the grain boundary and makes the grain boundarymore brittle, which leads to reduction of the toughness and strength.Therefore, Sr is preferably added 1.0% or less. In consideration of Srbeing a rare element, a preferable range is 0.8% or less and a morepreferable range is 0.5% or less from the practical point of view.

In the steam turbine casing, the main steam valve, and the steam controlvalve using the heat resisting cast steel according to the presentinvention, if the δ ferrite structure exists in a mixed state, thehigh-temperature creep rupture strength and the low-temperaturetoughness are reduced. Therefore, the heat resisting cast steelpreferably has the uniform tempered martensite structure. To obtain theuniform tempered martensite structure, the Cr equivalent expressed bythe following formula (1) must be held 10 or less. On the other hand,the Cr equivalent must be held 4 or more because the high-temperaturecreep rupture strength is reduced if the Cr equivalent is too low.

Cr equivalent (% by mass)=Cr+6Si+4Mo+1.5W+11V5Nb−40C−30N−30B−2Mn−4Ni−2Co   (1)

Reduction of P and S is effective in increasing the creep rupturestrength and the low-temperature toughness. Therefore, their contentsare preferably held as low as possible. Specifically, P and S are eachpreferably held 0.020% or less from the viewpoint of increasing thelow-temperature toughness. In particular, a preferable range of P is0.015% or less, a preferable range of S is 0.015% or less, a morepreferable range of P is 0.010% or less, and a more preferable range ofS is 0.010% or less.

Reduction of Sb, Sn and As is also effective in increasing thelow-temperature toughness, and therefore their contents are preferablyheld as low as possible. Specifically, Sb is preferably held 0.0015% orless, Sn is preferably held 0.01% or less, and As is preferably held0.02% or less from the viewpoint of the current level of thesteel-making technology. More preferably, Sb is held 0.0010% or less, Snis held 0.005% or less, and As is held 0.01% or less.

Because the steam turbine casing, the main steam valve, and the steamcontrol valve are exposed to high-pressure steam at 620° C. or above,high stress is caused therein due to inner pressure. From the viewpointof preventing creep rupture, therefore, the casing material is requiredto have the creep rupture strength at 620° C. and 10⁵ hours of 98 MPa ormore. Also, because thermal stress is caused at a relatively lowmaterial temperature when the turbine is started, the casing material isrequired to have the impact absorbed energy at room temperature of 29.4J or more from the viewpoint of preventing brittle fracture. To ensurehigher reliability, preferably, the creep rupture strength at 620° C.and 10⁵ hours is 108 MPa or more, and the impact absorbed energy at roomtemperature is 31.4 J or more.

In the steam turbine casing, particularly, the steel ingot is alarge-sized mass having weight of about 50 tons, and the highlydeveloped steel-making technology is required to produce a steel ingotincluding less defects. The high-strength heat resisting cast steelaccording to the present invention can be produced in the satisfactoryform through the steps of smelting, in an electric furnace, alloy rawmaterials with the objective composition, deaerating the smeltedmaterials by vacuum ladle refining, and casting the deaerated materialsinto a sand mold. By sufficiently refining and deoxidizing the materialsbefore the casting step, the cast steel can be obtained with lesscasting defects such as shrinkage. The above point is similarly appliedto the production of the main steam valve and the steam control valve.

Also, a large-sized steel ingot adapted for, e.g., the steam turbinecasing, which can be used in steam at 620° C. or above, through thesteps of annealing, at 1000-1150° C., the heat resisting cast steelobtained as described above; performing thermal normalizing of theannealed steel through steps of heating to 1000-1100° C. and subsequentquick cooling, and successively performing primary tempering at 550-750°C. and secondary tempering at 670-770° C. If the annealing temperatureand the normalizing temperature are 1000° C. or below, carbo-nitridescannot be obtained in the state of sufficient solid solution.Conversely, if those temperatures are too high, coarser crystal grainsare caused. Further, by performing two successive stages of tempering,the residual austenite structure can be completely decomposed and theuniform tempered martensite structure can be obtained. As a result ofproducing the high-strength heat resisting cast steel by theabove-described method, the steam turbine casing capable of being usedin steam at 620° C. or above is realized in which the creep rupturestrength at 620° C. and 10⁵ hours is 98 MPa or more, and the impactabsorbed energy at room temperature is 29.4 J or more.

Thus, the present invention can provide the ferrite-base heat resistingcast steel having high creep rupture strength at 620° C. and hightoughness at room temperature, which can be used for casings of an ultrasuper critical (USC) pressure steam turbine operated at temperaturesuntil 650° C.

Also, by using the heat resisting cast steel of the present inventionfor the casings of the USC pressure steam turbine operated attemperatures until 650° C. instead of the austenite heat resisting caststeel, those casings can be produced in accordance with the same designconcept as that in the past. Further, since the heat resisting caststeel of the present invention has a smaller thermal expansioncoefficient than the austenite heat resisting cast steel, otheradvantages are obtained in that quick start of the steam turbine can bemore easily performed and the casings are less susceptible to damagescaused by thermal fatigue.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between the W content and the10⁵-hour creep rupture strength (stress);

FIG. 2 is a graph showing the relationship between the W content and theimpact absorbed energy;

FIG. 3 is a graph showing the relationship between the Co content andthe 10⁵-hour creep rupture strength;

FIG. 4 is a graph showing the relationship between the Co content andthe impact absorbed energy;

FIG. 5 is a graph showing the relationship between [W/(Mo+0.5W)] and the10⁵-hour creep rupture strength;

FIG. 6 is a graph showing the relationship between [W/(Mo+0.5W)] and theimpact absorbed energy;

FIG. 7 is a graph showing the relationship between (Co/W) and the10⁵-hour creep rupture strength;

FIG. 8 is a graph showing the relationship between (Co/W) and the impactabsorbed energy;

FIG. 9 is a graph showing the relationship between [W/(Mo+0.5W)] and(Co/W);

FIG. 10 is a graph showing the relationship between the impact absorbedenergy (vertical axis) and the creep rupture strength (horizontal axis);

FIG. 11 is a graph showing the relationship between the creep rupturestrength (vertical axis) and the impact absorbed energy (horizontalaxis);

FIGS. 12A-12C are schematic views showing the structure of a specimenused in a weld cracking test;

FIG. 13 is a cross-sectional view showing the structure of ahigh-pressure steam turbine according to the present invention;

FIG. 14 is a cross-sectional view showing the structure of anintermediate-pressure steam turbine according to the present invention;

FIG. 15 is a cross-sectional view showing the structure of a high- andintermediate-pressure integral steam turbine according to the presentinvention; and

FIG. 16 is a cross-sectional view of a main steam valve and a steamcontrol valve according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The best mode for carrying out the present invention will be describedin detail below in connection with exemplary embodiments. It is to benoted that the present invention is not limited to those embodiments.

First Embodiment

Table 1, given below, shows chemical composition (% by mass) of the heatresisting cast steel according to the present invention. Assuming athick wall portion of a large-sized casing, a sample was prepared bysmelting 200 kg of raw materials in a high-frequency induction meltingfurnace, and casting the smelted materials into a sand mold with athickness of 200 mm, a width of 380 mm and a height of 440 mm atmaximum, thereby obtaining a cast ingot. In Table 1, samples No. 1-13represent the steel of the present invention, and samples No. 30-35represent the known comparative steels. Any of the samples was subjectedto annealing of 1050° C.×8 hours with subsequent furnace cooling,thermal normalizing performed through steps of heating and holding of1050° C.×8 hours and subsequent air cooling, primary tempering throughsteps of heating and holding of 680° C.×7 hours and subsequent aircooling, and secondary tempering through steps of heating and holding of710° C.×7 hours and subsequent air cooling.

TABLE 1 Cr W/ Equiv- (Mo + C Si Mn Ni Cr Mo W V Nb Co Others Al N B Oalent 0.5W) Co/W No. 1 0.14 0.4 0.50 0.51 10.5 0.4 2.1 0.2 0.08 1.0 —0.012 0.050 0.0005 0.007 8.10 1.4483 0.4762 No. 2 0.13 0.2 0.60 0.5210.2 0.2 2.5 0.2 0.07 1.2 — 0.008 0.040 0.0005 0.008 6.41 1.7241 0.4800No. 3 0.16 0.3 0.55 0.49 10.1 0.5 2.0 0.2 0.08 1.2 — 0.009 0.054 0.00140.005 5.98 1.3333 0.6000 No. 4 0.12 0.5 0.58 0.65 10.7 0.2 2.5 0.2 0.060.5 — 0.007 0.048 0.0052 0.004 9.59 1.7241 0.2000 No. 5 0.15 0.4 0.490.55 10.9 0.5 2.0 0.2 0.08 0.2 — 0.008 0.050 0.0068 0.006 9.62 1.33330.1000 No. 6 0.13 0.4 0.61 0.54 11.2 0.2 2.8 0.2 0.09 1.5 Sr0.3 0.0200.050 0.0021 0.008 6.11 1.7500 0.5357 No. 7 0.14 0.5 0.62 0.25 10.4 0.42.9 0.2 0.04 1.5 Nd0.3 0.016 0.034 0.0091 0.009 9.62 1.5676 0.5172 No. 80.14 0.4 0.57 0.12 10.1 0.4 2.6 0.2 0.03 1.4 0.024 0.058 0.0014 0.0108.55 1.5294 0.5385 No. 9 0.13 0.3 0.46 0.45 10.7 0.5 2.4 0.2 0.07 1.2Re0.7 0.031 0.035 0.0051 0.009 9.10 1.4118 0.5000 No. 10 0.14 0.6 0.570.35 9.5 0.6 2.1 0.2 0.04 1.1 Ta0.02 0.017 0.071 0.0006 0.014 8.561.2727 0.5238 No. 11 0.15 0.5 0.58 0.57 9.7 0.5 1.9 0.2 0.06 1.1 — 0.0130.065 0.0005 0.006 6.45 1.3103 0.5789 No. 12 0.12 0.6 0.59 0.55 10.4 0.42.1 0.2 0.08 1.0 — 0.010 0.042 0.0005 0.005 9.90 1.4483 0.4762 No. 130.10 0.5 0.54 0.52 10.2 0.7 2.0 0.2 0.07 1.4 — 0.008 0.058 0.0005 0.0049.84 1.1765 0.7000 No. 30 0.12 0.10 0.59 0.51 10.13 0.79 2.01 0.21 0.032.99 Ta0.03 0.012 0.04 0.0110 0.006 3.84 1.1198 1.4876 No. 31 0.11 0.120.58 0.50 10.05 0.77 2.06 0.20 0.04 3.02 — 0.031 0.03 0.0054 0.006 4.601.1444 1.4660 No. 32 0.10 0.20 0.30 0.50 10.0 0.40 1.80 0.15 0.057 3.5 —0.014 0.031 0.0020 0.007 2.05 1.3846 1.9444 No. 33 0.11 0.20 0.30 0.509.7 0.60 3.20 0.14 0.053 2.4 — 0.004 0.030 0.0120 0.008 6.85 1.45450.7500 No. 34 0.10 0.20 0.30 0.50 9.9 0.40 1.85 0.15 0.042 3.5 — 0.0240.027 0.0074 0.004 2.70 1.3862 1.8919 No. 35 0.10 0.20 0.30 0.50 9.61.10 1.21 0.14 0.043 3.6 0.026 0.026 0.0031 0.006 4.10 0.7097 2.9752

Table 2, given below, shows the test results of tensile strength at roomtemperature, V-notch Charpy impact absorbed energy at 20° C., creeprupture strength at 620° C.×10⁵ hours, and weld cracks.

In the samples No. 1-13 representing the steel of the present inventionin which B, Mo, W and Co are added in proper amounts and the values of[W/(Mo+0.5W)] and (Co/W) are set in the predetermined region, the creeprupture strength and the impact absorbed energy sufficiently satisfy theabove-described characteristics (i.e., the creep rupture strength at620° C.×10⁵ hours≧98 MPa and the impact absorbed energy at 20° C.≧29.4J) which are required for the high-temperature and high-pressure turbinecasing used at temperatures until 650° C.

TABLE 2 Tensile Reduction 10⁵-Hour Strength Elongation of Area vERupture Strength Weld Cracks MPa % % J MPa Cracked/Not Cracked No. 168.4 22.3 68.5 34.5 110 Not Cracked No. 2 71.1 20.1 59.8 31.4 115 NotCracked No. 3 72.4 20.5 62.8 29.8 109 Not Cracked No. 4 72.4 19.8 65.445.1 115 Not Cracked No. 5 70.9 20.9 62.1 31.5 107 Not Cracked No. 673.1 20.3 64.1 32.6 125 Not Cracked No. 7 73.8 20.8 62.7 30.8 122 NotCracked No. 8 72.4 22.0 65.3 29.9 115 Not Cracked No. 9 71.8 19.4 59.735.9 128 Not Cracked No. 10 70.9 18.7 64.7 30.5 107 Not Cracked No. 1169.8 20.5 65.3 35.7 104 Not Cracked No. 12 67.4 20.3 65.4 34.1 101 NotCracked No. 13 69.5 17.9 62.4 40.2 112 Not Cracked No. 30 71.2 20.5 65.430.4 87 Not Cracked No. 31 70.5 22.4 62.2 29.8 94 Not Cracked No. 3268.9 19.4 60.3 34.7 78 Not Cracked No. 33 78.4 17.4 55.2 11.2 112 NotCracked No. 34 70.4 18.9 58.4 34.2 87 Not Cracked No. 35 64.9 20.4 60.840.6 91 Not Cracked

FIG. 1 is a graph showing the relationship between the W content and the10⁵-hour creep rupture strength (stress). As shown in FIG. 1, in any ofthe comparative steel No. 32 in which the Nb content is 0.057%, No. 33in which the Nb content is 0.053%, No. 35 in which the Nb content is0.04%, No. 34 in which the Nb content is 0.04%, No. 31 in which the Nbcontent is 0.04%, and No. 30 in which the Nb content is 0.03%, the creeprupture strength is increased with an increase of the W content, and alarger effect is obtained with the higher Nb content. On the other hand,in the samples No. 1-13 representing the steel of the present inventionin which the Co content is 0.2-1.5%, it is apparent that any of thesteel samples of the present invention has higher creep rupture strengththan the known steel samples having the same Nb and W contents. If the Wcontent is 1.8% or less, the influence of the W content does not appear.However, if the W content exceeds 1.8%, the creep rupture strength isnoticeably increased. Further, in the samples No. 1-13 representing thesteel of the present invention, the creep rupture strength is noticeablyincreased with an increase of the W content at any of the Nb content ofabout 0.04% and the Nb content of about 0.08%.

FIG. 2 is a graph showing the relationship between the W content and theimpact absorbed energy. As a result of studying influences of W uponmechanical properties, it is apparent that, for the same Nb contents asthose in the above case, the impact absorbed energy is reduced as the Wcontent increases. However, in any of the samples No. 1-13 representingthe steel of the present invention in which the Co content is 0.2-1.5%,the impact absorbed energy is higher than the known steel samples havingthe same Nb and W contents. Also, in the steel samples of the presentinvention, reduction of the impact absorbed energy at room temperaturedoes not appear at the W content within 3.0%. Further, in the steelsamples of the present invention having the low Nb contents of0.03-0.06%, the impact absorbed energy at room temperature is notreduced with an increase of the W content.

FIG. 3 is a graph showing the relationship between the Co content andthe 10⁵-hour creep rupture strength. As shown in FIG. 3, in the knownsteel samples, it is apparent that, for the same Nb contents as those inthe above case, the creep rupture strength is reduced with an increaseof the Co content. On the other hand, in the low- and medium-Co steelsamples of the present invention in which the Co content is 0.2-1.5%,the creep rupture strength is increased with an increase of the Cocontent. Accordingly, it is apparent that, looking at both sides of aboundary corresponding to the Co content of 2%, any of the sampleshaving the Co content below 2% has higher creep rupture strength, whilein the side where the Co content is above 2%, the creep rupture strengthis conversely reduced with an increase of the Co content.

FIG. 4 is a graph showing the relationship between the Co content andthe impact absorbed energy. As shown in FIG. 4, any of the comparativesteel samples and the steel samples of the present invention has atendency to have higher impact absorbed energy with an increase of theCo content. However, the Co content in excess of 2% leads to theunsatisfactory result because the structure stability is reduced and the10⁵-hour creep rupture strength is also reduced as described above.

FIG. 5 is a graph showing the relationship between [W/(Mo+0.5W)] and the10⁵-hour creep rupture strength. As shown in FIG. 5, in the samples No.30-35 representing the comparative steels having the high Co contents,the 10⁵-hour creep rupture strength is slightly reduced with an increaseof [W/(Mo+0.5W)]. On the other hand, in the steel samples of the presentinvention, the 10⁵-hour creep rupture strength is increased with anincrease of [W/(Mo+0.5W)].

FIG. 6 is a graph showing the relationship between [W/(Mo+0.5W)] and theimpact absorbed energy. As shown in FIG. 6, in the samples No. 30-35representing the comparative steels having the high Co contents, theimpact absorbed energy is slightly reduced with an increase of[W/(Mo+0.5W)]. On the other hand, in the steel samples of the presentinvention, reduction of the impact absorbed energy with an increase of[W/(Mo+0.5W)] does not appear.

FIG. 7 is a graph showing the relationship between (Co/W) and the10⁵-hour creep rupture strength. As shown in FIG. 7, in the samples No.30-35 representing the comparative steels having the high Co contents,at (Co/W) of 0.75 or above, the 10⁵-hour creep rupture strength isreduced with an increase of (Co/W) and a reduction rate is steeper atthe higher Nb content. On the other hand, in the steel samples of thepresent invention in which (Co/W) is 1.0 or below, the influence of(Co/W) is small and change of the 10⁵-hour creep rupture strength issmall when (Co/W) is in the range of 0.1-0.70.

FIG. 8 is a graph showing the relationship between (Co/W) and the impactabsorbed energy. As shown in FIG. 8, in the known steel samples, at(Co/W) of 0.75 or above, the impact absorbed energy is increased as thevalue of (Co/W) increases, and the increase rate is steeper at thehigher Nb content. On the other hand, in the steel samples of thepresent invention in which (Co/W) is in the range of 0.1-0.70, change ofthe impact absorbed energy is small.

FIG. 9 is a graph showing the relationship between [W/(Mo+0.5W)] and(Co/W). By adjusting the value of [W/(Mo+0.5W)] to fall in the range of1.1-1.8 and the value of (Co/W) to fall in the range of 0.1-0.90, thecasing material made of the heat resisting cast steel is obtained withthe creep rupture strength at 620° C. and 10⁵ hours of 98 MPa or moreand the impact absorbed energy at room temperature of 29.4 J, whichcharacteristics are required when the steel is used for high- andintermediate-pressure inner casings of the high-temperature andhigh-pressure steam turbine operated at temperatures of 620° C. or aboveand pressures of 25 MPa or higher, as well as for casings of the mainsteam valve and the steam control valve.

FIG. 10 is a graph showing the relationship between the impact absorbedenergy (vertical axis) and the creep rupture strength (horizontal axis).As shown in FIG. 10, the impact absorbed energy is reduced as the creeprupture strength increases, and has the above-described correlation withrespect to the Nb content. Accordingly, it is apparent that the samplesNo. 1-13 representing the steel of the present invention have higherimpact absorbed energy than the comparative steel samples having thesame Nb contents and having the same creep rupture strengths.

FIG. 11 is a graph showing the relationship between the creep rupturestrength (vertical axis) and the impact absorbed energy (horizontalaxis). As shown in FIG. 11, the creep rupture strength is reduced as theimpact absorbed energy increases, and has the above-describedcorrelation with respect to the Nb content. Accordingly, it is apparentthat the samples No. 1-13 representing the steel of the presentinvention have higher creep rupture strength than the comparative steelsamples having the same Nb contents and having the same impact absorbedenergy.

FIGS. 12A, 12B and 12C are a front view, a side view, and across-sectional view, respectively, showing the shape and dimensions ofa specimen used in a weld cracking test made on the steel of the presentinvention. Weldability was evaluated by an inclined Y-groove weldcracking test in conformity with JIS 23158. Welding was performed bysetting each of the preheating temperature, the inter-pass temperature,and the post-heating start temperature to 150° C., and by using a coatedarc-welding electrode containing C in amount slightly smaller than thatin the base material (matrix), 0.5% or thereabout of Si, 1.5% orthereabout of Mn, 0.9% of Ni, 1.0% of Mo, the amount of each of thoseelements being slightly larger than that in the base material, as wellas Cr, Nb, V and N each in the same amount as that in the base material.Post-heat treatment was performed at 400° C.×30 minutes. As seen fromTable 2, the steel samples of the present invention caused no weldcracks and exhibited good weldability.

Thus, according to the first embodiment, the ferrite-base heat resistingcast steel having high creep rupture strength at 620° C. and hightoughness at room temperature can be obtained, and it can be suitablyused for the casings of the ultra super critical (USC) pressure steamturbine operated at temperatures until 650° C., the main steam valve,and the steam control valve.

Also, by using the heat resisting cast steel of the present inventionfor the casings of the USC pressure steam turbine operated attemperatures until 650° C., the main steam valve, and the steam controlvalve instead of the austenite heat resisting cast steel, those casingscan be produced in accordance with the same design concept as that inthe past. Further, since the heat resisting cast steel of the presentinvention has a smaller thermal expansion coefficient than the austeniteheat resisting cast steel, other advantages are obtained in that quickstart of the steam turbine can be more easily performed and the casingsare less susceptible to damages caused by thermal fatigue.

Second Embodiment

FIG. 13 is a cross-sectional view showing the structure of ahigh-pressure steam turbine using the high-strength heat resisting caststeel according to the present invention. FIG. 14 is a cross-sectionalview showing the structure of an intermediate-pressure steam turbineusing the high-strength heat resisting cast steel according to thepresent invention. The high-pressure steam turbine includes an innercasing 18, an outer casing 19 surrounding the inner casing 18, and ahigh-pressure rotor shaft 20 provided with high-pressure rotor blades 16implanted to it and disposed inside those casings. Theintermediate-pressure steam turbine includes an inner casing 21, anouter casing 22 surrounding the inner casing 21, and anintermediate-pressure rotor shaft 24 provided with intermediate-pressurerotor blades 17 implanted to it and disposed inside those casings.High-temperature and high-pressure steam obtained by a boiler passesthrough a main steam pipe and flows into a main steam inlet 28 through aflange and elbow 25 constituting a main steam section. The steam is thenintroduced to the rotor blade in the double-flow first stage through anozzle box 38. Stator nozzles are disposed corresponding to the rotorblades.

In this second embodiment, a steam turbine power plant is constituted bythe high-pressure steam turbine (HP) and the intermediate-pressure steamturbine (IP) which are connected in tandem. The HP has the double-flowfirst stage and includes eight stages in each side. Stator nozzles aredisposed corresponding to the rotor blades in both the sides. Each rotorblade is of the saddle-like dovetailed type and has double tenons.

The IP is used to rotate a generator in cooperation with the HP byutilizing steam obtained by heating the steam, which is exhausted fromthe HP, to 625° C. again by a reheater, and it is rotated at rotationspeed of 3000 rpm. Similarly to the HP, the IP has anintermediate-pressure inner compartment (casing) 21 and anintermediate-pressure outer compartment (casing) 22, and furtherincludes stator nozzle corresponding to the intermediate-pressure rotorblades 17. The intermediate-pressure rotor blades 17 are constitutedwith two flow sections each including six stages and are arranged insubstantially bilateral symmetry in the longitudinal direction of theintermediate-pressure rotor shaft 24.

In this embodiment, in both the HP and LP, the outer casings 19 and 22,the inner casings 18 and 21, and respective casings of a main steamvalve and a steam control valve (described later) were each made of thesample No. 4 cast steel in Table 1 described above in connection withthe first embodiment. Particularly, the inner casings were each producedthrough the steps of smelting, in an electric furnace, 50 tons of alloyraw materials with the objective composition, deaerating the smeltedmaterials by vacuum ladle refining, and casting the deaerated materialsinto a sand mold.

The obtained trial cast steel was successively subjected to annealing of1050° C.×10 hours with subsequent furnace cooling, thermal normalizingperformed through steps of heating and holding of 1050° C.×10 hours andsubsequent blast air cooling, primary tempering of 570° C.×12 hours, andsecondary tempering of 730° C.×12 hours. As a result of cutting andexamining the trial casing having the fully tempered martensitestructure, it was proved that the trial casing satisfied thecharacteristics (i.e., the creep rupture strength at 620° C. and 10⁵hours ≧98 MPa and the impact absorbed energy at room temperature ≧29.4J) required for the casing of the high-temperature and the high-pressureturbine operated at 620° C. and 25 MPa, and it caused no cracks in theabove-described weld cracking test.

Thus, according to the second embodiment, the ferrite-base heatresisting cast steel having high creep rupture strength at 620° C. andhigh toughness at room temperature can be obtained, and it can besuitably used for, e.g., the casings of the ultra super critical (USC)pressure steam turbine operated at temperatures until 650° C.

Also, by using the heat resisting cast steel of the present inventionfor the casings of the USC pressure steam turbine operated attemperatures until 650° C. instead of the austenite heat resisting caststeel, those casings can be produced in accordance with the same designconcept as that in the past. Further, since the heat resisting caststeel of the present invention has a smaller thermal expansioncoefficient than the austenite heat resisting cast steel, otheradvantages are obtained in that quick start of the steam turbine can bemore easily performed and the casings are less susceptible to damagescaused by thermal fatigue.

The steam turbine power plant of this embodiment further includes twolow-pressure steam turbines (LPs) which are connected in tandem and havesubstantially the same structure. In each LP, last-stage and other rotorblades are disposed in eight stages in each of the left and right sidesand are arranged in substantially bilateral symmetry. Stator nozzles aredisposed corresponding to the rotor blades. A low-pressure rotor shaftis made of forged steel having the fully tempered bainite structure of asuper-clean material containing 3.75% of Ni, 1.75% of Cr, 0.4% of Mo,0.15% of V, 0.25% of C, 0.05% of Si, and 0.10% of Mn, the balance beingFe. Any of the rotor blades and the stator nozzles in stages other thanthe last stage is made of 12%-Cr steel containing 0.1% of Mo.

The last-stage rotor blade has an airfoil height of 43 inches. Anairfoil of the long blade having the airfoil height of 43 inches,against which high-speed steam impinges, is coated with an erosionshield formed by joining a stellite sheet made of a Co-base alloy bywelding in order to prevent erosion caused by water droplets in thesteam.

The 43-inch long blade is produced through a series of steps of smeltingby the electroslag remelting process, forging, and heat treatment. Thelong blade is made of martensite steel containing 0.08-0.18% by mass ofC, 0.25% or less of Si, 0.90% or less of Mn, 8.0-13.0% of Cr, 2-3% ofNi, 1.5-3.0% of Mo, 0.05-0.35% of V, 0.02-0.20% of one or more of Nb andTa in total, and 0.02-0.10% of N. Further, the long blade exhibits thetensile strength at room temperature of 120 kgf/mm² or more and has thefully tempered martensite structure. As the mechanical properties of the43-inch long blade, more preferably, the tensile strength is 128.5kgf/mm² or more and the V-notch Charpy impact value at 20° C. is 4kgf-m/cm² or more.

The high-temperature and high-pressure steam turbine power plantaccording to this embodiment comprises mainly a coal firing boiler, oneHP, one IP, two LPs, a condenser, a condensing pump, a low-pressurefeedwater heater system, a deaerator, a booster pump, a feedwater pump,and a high-pressure feedwater heater system. More specifically, ultrahigh-temperature and high-pressure steam generated in the boiler entersthe HP in which motive power is produced. Then, the steam is reheated bythe boiler and enters the IP in which motive power is produced. Thesteam exhausted from the IP enters the LP in which motive power isproduced, followed by being condensed in the condenser. The condensedwater is sent to the low-pressure feedwater heater system and thedeaerator by the condensing pump. The feedwater deaerated in thedeaerator is sent to the high-pressure feedwater heater system by thebooster pump and the condensing pump. After the water temperature israised in the high-pressure feedwater heater system, the feedwater isreturned to the boiler. In the boiler, the feedwater is converted tohigh-temperature and high-pressure steam through an economizer, anevaporator and a superheater.

Instead of the power plant of this embodiment, a tandem compound powerplant may be constituted in which one HP, one IP, and one or two LPs areconnected in tandem to rotate one generator, each of HP, IP and LPhaving similar structures to those described above.

Thus, according to this second embodiment, the steam turbine power plantcan be obtained which has high thermal efficiency and is suitable foruse with, e.g., the steam turbine casings having the long-time creeprupture strength and toughness which are required under steamtemperature condition of 620-650° C.

Also, by using the heat resisting cast steel of the present inventionfor the casings of the USC pressure steam turbine operated attemperatures until 650° C., the main steam valve, and the steam controlvalve instead of the austenite heat resisting cast steel, those casingscan be produced in accordance with the same design concept as that inthe past. Further, since the heat resisting cast steel of the presentinvention has a smaller thermal expansion coefficient than the austeniteheat resisting cast steel, other advantages are obtained in that quickstart of the steam turbine can be more easily performed and the casingsare less susceptible to damages caused by thermal fatigue.

Third Embodiment

FIG. 15 is a cross-sectional view showing the structure of a high- andintermediate-pressure integral steam turbine used in a steam turbinepower plant with steam temperature of 620° C. and output capacity of 600MW. The power plant of this third embodiment is of the tandem compounddouble-flow type, and the last-stage blade height in the LP is 43inches. A rotation speed of 3000 rpm is obtained by the high- andintermediate-pressure integral steam turbine (HP-IP) and one LP (C) ortwo LPs (D). Components used in a high-temperature region are each madeof main materials shown in Table 1, which constitute the steel of thepresent invention. The steam temperature and pressure in thehigh-pressure section (HP) are 600° C. and 250 kgf/cm². In theintermediate-pressure section (IP), the steam temperature is heated to600° C. by a reheater and operation is performed at pressure of 45-65kgf/cm². Steam having temperature of 400° C. enters the low-pressuresection (LP) and is sent to a condenser under vacuum of 722 mmHg at 100°C. or below.

In the (HP-IP), the high-pressure side steam turbine includes an innercasing 18 and an outer casing 19 surrounding the inner casing 18,whereas the intermediate-pressure steam turbine includes an inner casing21 and an outer casing 22 surrounding the inner casing 21. A high- andintermediate-pressure integral rotor shaft 23 provided withhigh-pressure rotor blades 16 and intermediate-pressure rotor blades 17implanted to the rotor shaft is disposed inside those casings. Thehigh-pressure and high-temperature steam obtained by the boiler passesthrough a main steam pipe and flows into a main steam inlet 28 through aflange and elbow 25 constituting a main steam section. The steam is thenintroduced to the rotor blade in the first stage through a nozzle box38. The rotor blades are disposed in eight stages on the high-pressureside in substantially a left half in FIG. 15 and six stages on theintermediate-pressure side in substantially a right half in FIG. 15.Stator nozzles are disposed corresponding to the rotor blades. Eachrotor blade is of the saddle- or nearly π-like dovetailed type and hasdouble tenons.

In this third embodiment, in the (HP-IP), the outer casings 19 and 22and the inner casings 18 and 21, and respective casings of a main steamvalve and a steam control valve (described later) were each made of thesample No. 4 cast steel in Table 1 described above in connection withthe first embodiment. As in the second embodiment, those casings wereeach produced through the steps of smelting, in an electric furnace, 50tons of alloy raw materials with the objective composition, deaeratingthe smelted materials by vacuum ladle refining, and casting thedeaerated materials into a sand mold.

Further, in this third embodiment, those casings also had the fullytempered martensite structure. As a result of cutting and examining eachtrial casing, it was proved that the trial casing satisfied thecharacteristics (i.e., the creep rupture strength at 620° C. and 10⁵hours ≧98 MPa and the impact absorbed energy at room temperature ≧29.4J) required for the casing of the high-temperature and the high-pressureturbine operated at 620° C. and 25 MPa, and it caused no cracks in theabove-described weld cracking test.

Thus, according to the third embodiment, the ferrite-base heat resistingcast steel having high creep rupture strength at 620° C. and hightoughness at room temperature can be obtained, and it can be suitablyused for, e.g., the casings of the ultra super critical (USC) pressuresteam turbine operated at temperatures until 650° C.

Also, by using the heat resisting cast steel of the present inventionfor the casings of the USC pressure steam turbine operated attemperatures until 650° C. instead of the austenite heat resisting caststeel, those casings can be produced in accordance with the same designconcept as that in the past. Further, since the heat resisting caststeel of the present invention has a smaller thermal expansioncoefficient than the austenite heat resisting cast steel, otheradvantages are obtained in that quick start of the steam turbine can bemore easily performed and the casings are less susceptible to damagescaused by thermal fatigue.

In the steam turbine power plant of this embodiment, one or twolow-pressure steam turbines (LPs) are connected in tandem and havesubstantially the same structure as that in the second embodiment. Ineach LP, last-stage and other rotor blades are disposed in eight stagesin each of the left and right sides and are arranged in substantiallybilateral symmetry, thus enabling power generation with an outputcapacity of 1050 MW. Further, the last-stage rotor blade has an airfoilheight of 43 inches and is made of the 12%-Cr steel as in theabove-described case. A low-pressure rotor shaft, the rotor blades andthe stator nozzles in stages other than the last stage, and inner andouter casings are each produced in a similar manner.

An airfoil of the long blade having the airfoil height of 43 inches,against which high-speed steam impinges, is coated with an erosionshield formed by joining a stellite sheet made of a Co-base alloy bywelding in order to prevent erosion caused by water droplets in thesteam.

The 43-inch long blade is produced, as in the second embodiment, througha series of steps of smelting by the electroslag remelting process,forging, and heat treatment. The long blade has the fully temperedmartensite structure exhibiting the tensile strength at room temperatureof 120 kgf/mm² or more, preferably 128.5 kgf/mm² or more, and theV-notch Charpy impact value at 20° C. of 4 kgf-m/cm² or more.

According to this third embodiment, the steam turbine power plant can beobtained which has high thermal efficiency and is suitable for use with,e.g., the steam turbine casings having the long-time creep rupturestrength and toughness which are required under steam temperaturecondition of 620-650° C.

Fourth Embodiment

FIG. 16 is a cross-sectional view of a main steam valve and a steamcontrol valve used in a steam turbine power plant, which are formed inan integral structure. In this fourth embodiment, the main steam sentfrom the boiler to the high-pressure steam turbine and the high- andintermediate-pressure integral steam turbine in the steam turbine powerplant of the second or third embodiment is supplied to a main steamvalve 32 through a main steam inlet 34 and the amount of steam suppliedthrough a steam outlet 35 is controlled by a steam control valve 33.

In this embodiment, a casing of each of the main steam valve and thesteam control valve was made of the sample No. 4 cast steel in Table 1described above in connection with the first embodiment and was producedthrough the steps of smelting, in an electric furnace, alloy rawmaterials with the objective composition, deaerating the smeltedmaterials by vacuum ladle refining, and casting the deaerated materialsinto a sand mold. The obtained trial casing was subjected to heattreatment and a weld cracking test in a similar manner to those in thesecond embodiment.

As a result, it was proved that the trial casing satisfied thecharacteristics (i.e., the creep rupture strength at 620° C. and 10⁵hours ≧98 MPa and the impact absorbed energy at room temperature ≧29.4J) required for the casing of the high-temperature and the high-pressureturbine operated at 620° C. and 25 MPa, and it caused no cracks in theabove-described weld cracking test.

Thus, according to the fourth embodiment, the ferrite-base heatresisting cast steel having high creep rupture strength at 620° C. andhigh toughness at room temperature can be obtained, and it can besuitably used for the respective casings of the main steam valve and thesteam control valve in the ultra super critical (USC) pressure steamturbine operated at temperatures until 650° C. In other words, thefourth embodiment can also provide similar advantages to those in theabove-described embodiments.

1. A steam turbine casing made of a high-strength heat resisting caststeel containing 0.06-0.16% by mass of C, 0.1-1% of Si, 0.1-1% of Mn,10.0-11.0% of Cr, 0.1-1.0% of Ni, 0.7% or less of Mo, 2.0-2.5% of W,0.05-0.3% of V, 0.01-0.15% of one or more of Nb, Ta and Zr in total,0.2-1.2% of Co, 0.01-0.08% of N, and 0.0005-0.01% of B, and 0.0005-0.03%by mass of AI and 0.004-0.01% of O as well as at least one of 1.5% bymass or less of Re, 0.5% or less of Nd, and 1.0% or less of Sr, thebalance being Fe and unavoidable impurities, and wherein in orthogonalcoordinates expressed by the relationship between [W/(Mo+0.5W)] and(Co/W), values of [W/(Mo+0.5W)] and (Co/W) are not larger than thevalues represented by linear lines interconnecting a coordinate point A(1.1, 0.90), a coordinate point B (1.5, 0.55), and a coordinate point C(1.8, 0.55).
 2. A steam turbine comprising a rotor shaft includingimplanted rotor blades, an inner casing including stator nozzlesimplanted corresponding to said rotor blades and covering said rotorshaft including said implanted rotor blades, and an outer casingcovering said inner casing, wherein said inner casing is constituted bythe steam turbine casing according to claim
 1. 3. A main steam valve forcontrolling supply and stop of main steam obtained by a boiler withrespect to a steam turbine, wherein a casing of said main steam valve ismade of the high-strength heat resisting cast steel according toclaim
 1. 4. A steam control valve for controlling a supply amount ofmain steam obtained by a boiler through a main steam valve whichcontrols supply and stop of the main steam with respect to a steamturbine, wherein a casing of said steam control valve is made of thehigh-strength heat resisting cast steel according to claim
 1. 5. A steamturbine power plant including any of a set of a high-pressure steamturbine, an intermediate-pressure steam turbine, and two low-pressuresteam turbines connected in tandem, and a set of a high- andintermediate-pressure integral steam turbine and a low-pressure steamturbine, wherein at least one of said high-pressure steam turbine, saidintermediate-pressure steam turbine, and said high- andintermediate-pressure integral steam turbine is constituted by the steamturbine according to claim
 2. 6. A steam turbine power plant includingany of a set of a high-pressure steam turbine, an intermediate-pressuresteam turbine, and two low-pressure steam turbines connected in tandem,and a set of a high- and intermediate-pressure integral steam turbineand a low-pressure steam turbine, said power plant further including amain steam valve for controlling supply and stop of main steam obtainedby a boiler and a steam control valve for controlling a supply amount ofthe main steam through a main steam valve, wherein at least one of saidmain steam valve and said steam control valve is constituted by the mainsteam valve and the steam control valve according to claim 3.